When working with various toxic and hazardous materials including radioactive samples and strong chemical components in laboratory or industrial conditions it is essential that care be taken to insure that such materials are safely contained so that persons working with these materials are protected from direct exposure to the materials or any toxic vapors or air borne particles generated by the handling of such materials. When working with relatively small to moderate samples of radioactive chemicals and other hazardous materials it has been the practice to provide a working area which is generally isolated from the surrounding environment.
In laboratory environments the containment or isolation of hazardous and toxic laboratory chemicals is generally accomplished by providing a work area which is enclosed or covered with a hood with access to the work area being permitted only by means of openings which allow a chemist or laboratory technician to extend their arms into the work area. Because the entry openings to such work stations also provide an avenue by which hazardous materials and fumes could escape or be vented to the surrounding area, it has been the practice to provide a positive air flow through the openings and into the work area. By creating a continuous ingress of air into the covered work area, airborne contaminants are prevented from escaping therefrom.
In most laboratory environments, harmful or hazardous materials are handled safely within a laboratory fume hood. The hood captures contaminants and prevents them from escaping into the laboratory by utilizing an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that the inhalation of and contact with the contaminants or toxic components are minimized. Access to the interior of the hood is through one or more openings which are closed with a sash which typically slides up and down to vary the opening of the hood.
The velocity of air flow through the hood opening is called the face velocity. The more hazardous the material being handled, the higher the recommended face velocity. Guidelines have been established relating face velocity to toxicity. Generally, minimum face velocities for laboratory fume hoods are 75 to 150 feet per minute, depending on the application.
When the operator is working in the hood, the sash is open to allow free access to the materials inside. The sash may be open partially or fully, depending on the operations to be performed in the hood. While fume hood and sash sizes vary, the opening provided by the fully open sash is on the order of 10 square feet. Thus, the maximum air flow which the blower must provide is typically on the order of 750 to 1500 cubic feet per minute. The sash is closed when the hood is not being used by an operator. It is common to store hazardous or toxic materials inside a hood when the hood is not in use, and a positive air flow must therefore be maintained to exhaust contaminants from such materials even when the hood is not in use and a sash is closed.
In larger laboratory environments and particularly in industrial settings hazardous or toxic materials are often handled in such large quantities that they are handled in ventilated chambers, booths or walk-in exhaust hoods into which workers enter to work with and handle the hazardous or toxic materials. Such walk-in exhaust hoods, chambers and booths are conventionally utilized for such processes as paint spraying, welding, grinding, cutting, metallizing, drum filling operations and the like. It is estimated that there are about 50,000 walk-in exhaust hoods, chambers or booths used in the United States alone.
Efforts to insure proper ventilation and containment of hazardous or toxic materials when utilizing fume hoods or walk-in ventilated chambers or booths have generally focused on methods and apparatus which maintain constant positive air flow, methods and apparatus designed to regulate and vary ventilation air flow, methods and apparatus to create laminar air flow and various exhaust filtering systems to contain hazardous and toxic materials.
In spite of the advances made in controlling hazardous or toxic materials in fume hoods and walk-in ventilated work environments, there remains an unsolved problem associated with the formation of eddies as a result of the exhaust air flow through these work environments. An eddy is a feature of steady turbulent flow in which a circulating pattern develops downstream and adjacent to an obstacle. In a typical ventilation system, the eddy associated with a person will contribute to that person's exposure to gases, vapors or dust if these materials are released in the eddy. The circulating flow will carry the material back to the person's breathing zone.
Other obstacles will also have associated eddies which cause materials released downstream in a primary ventilating flow to be carried to upstream points. This effect is generally unintended and can decrease the ventilation system's effectiveness nearly to the point of negating it. In order to protect persons working in ventilated exhaust chambers, the formation of eddies needs to be controlled. Only by eliminating or reducing the strength of the eddy can a person experience the intended effect of a ventilating system and be insured of a contaminate free breathing zone.
The present invention is an improvement over prior known fume hoods, walk in ventilated work areas and the like which provides for highly effective ventilation by controlling and reducing eddy formation.